This invention relates generally to inkjet printheads and methods for fabricating inkjet printheads, and more particularly, to methods for improving reliability and increasing useful life of an inkjet printhead.
There are known and available commercial printing devices such as computer printers, graphics plotters and facsimile machines which employ inkjet technology, such as an inkjet pen. An inkjet pen typically includes an ink reservoir and an array of inkjet printing elements. The array is formed by a printhead. Each printing element includes a nozzle chamber, a firing resistor and a nozzle opening. Ink is stored in the reservoir and passively loaded into respective firing chambers of the printhead via an ink refill channel and respective ink feed channels. Capillary action moves the ink from the reservoir through the refill channel and ink feed channels into the respective firing chambers. The printing elements are formed on a common substrate. Printer control circuitry outputs respective signals to the printing elements to activate a firing resistor. In response the firing resistor heats the ink causing an expanding vapor bubble to form. The bubble forces ink from the nozzle chamber out the nozzle opening. A nozzle plate adjacent to the barrier layer defines the nozzle openings. The geometry of the nozzle chamber, ink feed channel and nozzle opening defines how quickly a corresponding nozzle chamber is refilled after firing.
To achieve high quality printing ink drops or dots are accurately placed at desired locations at designed resolutions. It is known to print at resolutions of 300 dots per inch and 600 dots per inch. Higher resolutions also are being sought. One of the obstacles to achieving high quality printing with inkjet technology is failed, blocked or otherwise defective inkjet nozzles. In a thermal inkjet printhead one source of nozzle failure is thin film failure. In fabricating inkjet printheads, inkjet nozzles are formed on a silicon die from various layers of a thin film structure. The thin film structure is deposited on the die to define firing resistors, wiring lines and various passivation and insulation layers. The nozzle chambers and firing resistors define respective nozzles or printing elements. The thin film structure includes an array of printing elements with various areas between printing elements. During the printing cycle of a thermal inkjet printhead, local areas of the thin film are exposed to substantial changes in temperature due to the heating and cooling of the firing resistors. These changes in temperature cause severe thermal stresses on the thin film structure. As the printhead approaches the end of its useful life, it is expected that the thin film structure may crack or delaminate. For example, the top layer of the thin film structure typically is formed by a tough layer having a high hardness factor and a high ductility rating. This layer is exposed to high pressures from the collapsing bubble after ink ejection. Damage from continued exposure to such activity is referred to as cavitation damage.
Variations in the thermal coefficients of expansion of the thin film layers cause thermal stresses which may ultimately lead to delamination. It is failures such as this delamination which curtail a printhead's rated useful life. Another exemplary failure is cracking of a layer of the thin film structure. One or more adjacent resistors may fail from a crack or delamination of thin film layers. Further, with continued use of the printhead after a failure continued thermal stresses cause the crack to increase in length (i.e., propagate) or cause the delamination to increase in area (i.e., propagate). The result is a failure of additional nozzles.